The present invention relates to operational amplifiers and, more particularly, to integrated circuit amplifiers having output stages for providing increased output voltage swings.
Typical amplifier output stages utilize a pull up and a pull down transistor which are cascaded between positive and negative voltage supply rails for providing an output that is taken at the interconnection of the two transistors as is understood. Generally an emitter follower configured transistor is coupled between the bases of the pull up and pull down transistors which receives an input signal supplied from the input or middle stage. A quiescent bias current is supplied directly to the base of the pull up transistor and through the emitter of the aforementioned emitter follower transistor to the base of the pull down transistor. Due to this conventional configuration, a problem occurs in the prior art since the output stage voltage swing is thereby limited within a range of two base-emitter voltages (2 V.sub.BE) plus two collector-emitter saturation voltages V.sub.CE(SAT) of the two supply rail voltages. Since a typical V.sub.CE(SAT) value is 0.3 volts and a typical V.sub.BE value is 0.7 volts, the output voltage swing may be reduced by two volts from the algebraic difference of the two supply rail voltages. Thus, for a voltage supply rail difference range of two volts, the output voltage may swing 100 millivolts or less which is too small for most applications. However, if the two base-emitter voltages could be obviated, the output voltage swing could be increased by as much as 1.4 volts thereby achieving an output swing practical for even low supply voltages.
Prior art has made at least one attempt at providing an amplifier output stage having larger output voltage swings and is fully described in pending U.S. application Ser. No. 442,261 filed on Nov. 27, 1989 entitled "AMPLIFIER OUTPUT STAGE", the teaching of which is incorporated herein by reference made thereto. However, the above noted prior art requires the use of a low impedance voltage supply to obtain the increased output voltage swing. Furthermore, generating a low impedance voltage source can be difficult especially at high frequencies.
Hence, a need exists for an amplifier output stage circuit for providing increased output voltage swings with minimum components and optimum performance.